Conventionally, a six-level drive system is adopted in liquid crystal display devices of a simple-matrix type (including liquid crystal display devices of the same drive system such as MIM (Metal Insulator Metal), etc.). In the six-level drive system, a liquid crystal display panel is driven by supplying a potential of six levels to a liquid crystal driver from a drive voltage generating device. Therefore, in order to drive portable devices using batteries, it is required to set the voltage to be supplied to the liquid crystal driver higher than a voltage of a self-contained battery, and also to have potentials of six levels.
First, as a technique for achieving a voltage higher than a voltage from a power source, for example, a method for transferring a charge in a condenser may be used. This technique for achieving a higher voltage is enabled, for example, from a circuit shown in FIG. 3 of "voltage boosting circuit for electronic watches" disclosed in Japanese Examined Patent Publication No. 6424/1993 (Tokukohei 5-6424) and from a circuit shown in FIG. 1 of "up-voltage circuit" disclosed in Japanese Examined Patent Publication No. 49822/1987 (Tokukosho 62-49822).
The technique for achieving a higher voltage than the voltage of the power source by transferring the charge in the condenser also can be achieved from a Cockcroft circuit shown in FIG. 2 of "portable digital electronic device" disclosed in Japanese Laid-Open Patent Publication No. 74120/1973 (Tokukaisho 48-74120), a Schankel circuit shown in FIG. 3 of "digital electronic device" disclosed in Japanese Laid-Open Patent Publication No. 44781/1974 (Tokukaisho 49-44781), and Cockcroft and Schankel circuits shown in FIG. 2 of "digital electronic device" disclosed in Japanese Laid-Open Patent Publication No. 35074/1974 (Tokukaisho 49-35074).
However, all of the described voltage boosting circuits are designed for achieving a higher voltage than an original voltage generated from the power source and a potential of one level using a single power source and a capacitor. Therefore, it is difficult to modify the circuits so as to supply potentials of six levels to the liquid crystal driver.
In order to solve the above problem, as another technique for achieving potentials of six levels from the single power source, for example, as shown in FIG. 10, a method for supplying a voltage divided by a resistance R to a liquid crystal driver IC through an operational amplifier 101 which is voltage follower connected has been proposed. (Details are shown in FIG. 6.18, page 403 of the handbook for the liquid crystal device of the 142nd meeting of Nippon Gakujutsu Shinkokai published by Nikkan Industrial Newspaper Publishing company, and sections related to "T6A04" of IC data sheet of Toshiba Co., Ltd.) A variable resistance VR shown in FIG. 10 is provided for adjusting the contrast.
In the described arrangement where the operational amplifier 101 is voltage follower connected, current flowing in a bleeder resistance can be significantly reduced compared with the arrangement where the voltage is simply divided by the resistance, thereby improving the accuracy of the output voltage.
However, the described conventional arrangement has the following factors of increasing power consumption, and the conventional techniques have not yet found a satisfactory solution to this problem.
(1) As a supply power source to the operational amplifier 101, a voltage between output terminal voltages V.sub.0 and V.sub.5 is used, and each difference between an input terminal voltage and output terminal voltage V.sub.1 -V.sub.4 is large. Therefore, in operating at a constant voltage, the difference in the voltage is absorbed by the operational amplifier 101 as heat as in the case of a series regulator.
(2) In addition to the energy supply from output terminal voltages V.sub.1 through V.sub.4, the operational amplifier 101 also absorbs a charge stored in a component of a condenser of liquid crystal. Therefore, the charge which is to be stored as electronic energy is also absorbed by the operational amplifier 101 as heat.
(3) In the case where displayed data greatly changes, such as a display which inverts ON/OFF at every line, a large control current is required for the constant maintenance of the output from the operational amplifier 101 following each change.